1. Field of the Invention
The present invention relates to rotors, such as those used in compressors, fans and turbines. More specifically, to an apparatus for locking side entry blades into such rotors.
2. Description of the Prior Art
Compressors, fans, turbines and like machinery employ rotors to which a plurality of blades are affixed. Such blades are arranged into one or more rows spaced axially along the rotor, the blades in each row being circumferentially arrayed around the periphery of the rotor.
As a result of the high steady and vibratory forces imposed on the blades during operation, the method of attaching the blades to the rotor requires careful design. One method of attachment employs approximately axially extending grooves formed in the rotor periphery. The shape of the grooves may be that of a fir-tree, semi-circle, inverted T, or some variation thereof. Each blade has a corresponding root portion at its base which is closely profiled to match the shape of the rotor grooves. Each blade is retained in the rotor by sliding the root of the blade into a rotor groove. Blades affixed to the rotor in this manner are referred to as side entry blades. As a result of the close match in the size and shape of the blade root and the rotor groove, motion of the blade in the tangential and radial directions is closely restrained. However, restraint of the blade in the axial direction, referred to as locking, requires a separate device. In the past, a variety of locking devices have been devised. Generally they can be divided into two categories depending on the location of the point of fixity.
The first category of locking devices applies to blades in which a platform is formed at the base of each blade airfoil, the platforms of adjacent blades abutting one another thereby forming a ring surrounding the periphery of the rotor. In such arrangements the locking device is usually employed at the periphery of the rotor. One approach, disclosed in U.S. Pat. No. 4,676,723, involves a tangential locking pin which straddles a groove in the rotor periphery and a mating groove in the underside of the blade platform. A second approach, disclosed in U.S. Pat. No. Nos. 2,867,408 and 2,843,356 and Swiss Patent No. 313,027, involves a locking plate, the lower portion of the plate resides in a tangential slot in the rotor periphery and the upper portion in a slot in the edge of the platform. A third approach, disclosed in U.S. Pat. No. 3,202,398, employs a locking plate which resides in an axial channel in the rotor periphery and features tabs on the ends of the plate which can be bent against the front and rear faces of the platform. A fourth approach, disclosed in U.S. Pat. No. 3,001,760, relies on a spring clip residing, at its base, in a tangential slot in the rotor periphery, and at its upper portion, in a radially aligned matching slot in the edge of the blade platform. In each of these approaches the retention of the locking device in a simple groove or slot is made possible by the cooperation of the abutting platform of the adjacent blade.
The second category of locking devices applies to blades without abutting platforms at the base of the airfoil and, hence, which cannot rely on the platforms to retain the locking device. In this arrangement, the locking device is usually employed at the bottom of the rotor groove. One approach, disclosed in Japanese Patent No. 54-130710, involves a locking plate which resides in an axial channel in the bottom of the groove and features tabs at both end of the locking plate which can be bent against the upstream and downstream faces of the blade root. A second approach, disclosed in U.S. Pat. No. 2,753,149, utilizes a rivet disposed in mating axial grooves in the base of the blade root and the bottom of the rotor groove. A third approach, disclosed in U.S. Pat. No. 3,759,633, utilizes balls disposed in mating semi-spherical depressions in the base of the blade root and the bottom of the rotor groove. A fourth approach, disclosed in U.S. Pat. No. 4,466,776, employs two tangential keys disposed in slots in the front and rear of the base of the blade root, the key being retained by tab-like projections emanating from its ends which are bent against the sides of the root.
The compressor rotors of gas turbines designed by the assignee of the present invention incorporate blades in which the airfoils emanate directly from the blade roots without intervening platforms. Hence, locking devices of the aforementioned first category, which rely on cooperation of the blade platforms to retain the locking devices, cannot be utilized. Instead, in the past, axial motion was restrained by a radially oriented spring and pin. In this approach each blade is installed by first disposing a spring in a hole in the bottom of the rotor groove and compressing the spring by forcing a pin into the hole on top of the spring. The blade root is slid into the groove and is locked when a slot, machined in the bottom of the root, passes over the pin, allowing the spring force to drive the pin partially out of the hole and into the slot. Blades are removed by applying an axial force to the blade root sufficient to shear the pin in half, allowing the blade to be withdrawn.
However, this approach suffers from several disadvantages. Firstly the locking device is hidden from view and its correct installation cannot be ascertained visually once the blade is inserted into the groove. Since there may be well over 1,000 blades in each rotor, this disadvantage makes inspection of the rotor for proper locking difficult and time-consuming. However, a single unlocked compressor blade, should it come loose in service, may result in substantial damage to the rotating blades and stationary vanes of the compressor and render the gas turbine unavailable for use until repaired. It should be noted that many of the locking devices utilized in the prior art suffer from a similar disadvantage.
A second disadvantage occurs because the bottom of the groove is a highly stressed region of the rotor and the presence of the hole serves to concentrate these stresses, thereby exacerbating the potential for cracking.
A third disadvantage concerns the strength of the locking device. As explained below, pins have been known to fail in service, resulting in unlocking of the blades.
During full speed operation the blades are urged axially forward by the pressure rise across the row of blades. The centrifugal force on the blades is very high however. Hence there is more than adequate frictional resistance in the blade roots to prevent them from sliding forward. However, when a gas turbine is shut down, its rotor is not allowed to come to rest immediately. Instead the rotor is usually rotated at low speed until it cools sufficiently to prevent gravity from forming a bow in the hot rotor since such a bow would result in high vibration during the next start up. This cooling time may be in the order of several days. During the cooling period, distortion may occur in the compressor cylinder due to non-uniformities in the temperature distribution within the cylinder, causing the tips of the rotating blades to contact the cylinder, a phenomenon known as blade tip rubbing. Since the compressor cylinder converges slightly as it extends rearward, to accommodate the reduced flow area required by the air as it undergoes compression, the tip rubbing gives rise to an axial force tending to urge the blades forward. Since during the cooling period the centrifugal force on the blades is nil, there is little frictional resistance to sliding in the groove. Consequently, the axial force imparted by the tip rubbing is transmitted to the pin. However, the pins must be weak enough to allow them to be sheared so that the blades can be removed, as previously explained, without damaging the holes in the rotor grooves or the slots in the blade roots in which they reside. Hence, if the tip rubbing is severe, it may result in shearing the pin in half, thus unlocking the blade. As explained previously, an unlocked blade may result in significant compressor damage.
This third disadvantage is exacerbated on recently designed compressors owing to the necessity for coating the blade roots with a lubricant to avoid fretting fatigue cracking of the blade root or rotor groove as a result of vibratory loading on the blades. The lubricant coating reduces the coefficient of friction between the root and groove, thus reducing the severity of tip rubbing required to shear a locking pin.
It should be noted that the other locking devices described as being in the second category, and therefore applicable to rotors whose blades do not feature abutting platforms, suffer from a similar limitation in the ability of the locking device to withstand a large axial force induced by tip rubbing.
Lastly, many of the locking schemes utilized in the prior art, such as disclosed in aforementioned U.S. Pat. Nos. 4,676,723; 2,867,408 and 2,843,356; Swiss Patent No. 313,027 and Japanese Patent No. 54-130710 require that the last blade, or next to the last blade, installed be of a special type. Such a requirement increases the quantity of blades which must be stocked in inventory and is, therefore, to be avoided.
It is therefore desirable to provide an apparatus for locking side entry blades, of the type without abutting platforms, which allows for visible inspection of the locking devices, is capable of withstanding large axial forces without loss of locking function and which allows removal of the blades without damage to the blades or rotor.